Most networks are organized as a series of layers, each one built upon its predecessor. The purpose of each layer is to offer services to the higher layers, shielding those layers from implementations details. Between each pair of adjacent layers there is an interface that defines those services.
The International Standards Organization has developed a layered network architecture called the Open Systems Interconnection (OSI) Reference model that has seven protocol layers: application, presentation, session, transport, network, data link, and physical.
The function of the lowest level, the physical layer, is to transfer bits over a communication medium. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames.
The interface between the data link layer and the physical layer includes a medium access control device, called a MAC device, and physical layer signaling control device, called a PHY device. The purpose of a MAC device and the PHY device is to ensure two network stations are communicating with the correct frame format and protocol.
In wireless local area networks (WLANs), a radio is the physical device, and free space is the physical communications medium. IEEE 802.11 is a proposed standard for WLANs that defines the communication protocol between a MAC device and a radio, the PHY device. WLAN data communication protocol requires that each data frame transferred between the MAC and the PHY devices have a PHY header, a MAC header, MAC data, and error checking fields. The PHY header includes a preamble that is used to indicate the presence of a signal, unique words, frame length, etc. The MAC header includes frame control, duration, source and destination address, and data sequence number, etc.
Although draft standard 802.11 defines the logical PHY/MAC interface, 802.11 does not define the physical interface between a MAC device and a PHY device. For example, assuming that the draft standard provides that a byte of information is to be passed from the MAC device to the PHY, the draft standard does not provide how the byte is be transferred. That is, the draft standard does not provide a physical definition as to whether the byte is passed as eight bits in parallel, or as one bit in serial using eight clock cycles. In addition to the draft WLAN standard, many proprietary WLANs exists that define various different physical interfaces between the MAC device and the PHY device.
Due to the lack of a coherent WLAN standard, different radio vendors incorporate various portions of the physical layer function inside the radio. Some radios, for example, are capable of generating the PHY data, while other radios require that the MAC device generate the PHY data. Therefore, current MAC devices must be tailored to meet the requirements of a specific radio.
Accordingly, what is needed is a flexible MAC interface that provides compatibility between a MAC device and a radio, regardless of the physical layer functionality of the radio.